The internment of people of Japanese descent in America during WWII was caused by two things.

First, it was caused by a very understandable fear for the security of the country. Japan had managed to pull off the attack on Pearl Harbor, which no one had thought was possible. The idea that they might attack the West Coast while the US military was still weak was not absurd.

Second, it was caused by racism. There was still a strong strand of racism in American society at this time. For example, Asians were not allowed to become naturalized citizens, which is why the Issei were not citizens. This racism led Americans to be willing to intern all people of Japanese descent without trying to determine individual guilt or innocence.

Internment of Japanese Americans was caused by a number of factors which include as mentioned by Pohnpei397:

Racism

War Hysteria

Fear and suspicion

There was profound racism against the American Japanese both from the society and some government policies. White farmers in the West Coast were highly prejudicial against their Japanese counterparts and the attack on Pearl Harbor offered them an opportunity to condemn and take away the farms owned by people of Japanese origin. Such groups instigated and fully supported the internment camps to enable them reach their objectives.

The government considered sabotage and espionage as activities that led to the success of the attack. People of Japanese decent were collectively viewed as supporters of the attack considering the first generation of Japanese settlers were not American citizens and owed allegiance to their mother country.

War hysteria led to growing fear propagated by provocative journalism among the society in general. This led to increasing pressure on the government to detain people of Japanese decent regardless of their existent or nonexistent role in the attack.